Support extending with custom providers

[seroperson]

Hello and thank you for your great plugin!
I'm trying to use it with some exotic LLMs, which are not supported out-of-box and it's kind a hard thing to do.

For example, I need to edit payload generation logic and I can do it by overriding prepare_payload lua function (link). There is no problem with it.
But I also need to edit response parsing function, which is defined locally (link) and can't be overriden.

To resolve my problem, I think it would be enough to move parsing function to global scope, but also maybe it would be good to re-visit this API with custom providers in mind.